sell someone a pup
sell someone a pupswindle someone, especially by selling them something that is worthless. British informal
This phrase originated in the early 20th century; the idea behind it is presumably that of dishonestly selling someone a young and inexperienced dog when an older, trained animal had been expected.
1930 W. Somerset Maugham Cakes and Ale The public has been sold a pup too often to take unnecessary chances.